The term hydrous calcium silicate denotes a crystalline compound formed by the reaction of lime (CaO), silica (SiO.sub.2), and water (H.sub.2 O). The two hydrous calcium silicates that generally are of interest are: tobermorite having the formula 4CaO.5SiO.sub.2. 5H.sub.2 O; and xonotlite, having the formula 5CaO.5SiO.sub.2.XH.sub.2 O. Hydrous calcium silicate products often are used as heat insulation materials.
Methods for reacting and drying a molded aqueous slurry of reactive cementitious constituents and reinforcing fibers to form hydrous calcium silicate insulation products are known in the art. One such method includes placing a molded slurry of the reactive cementitious constituents and reinforcing fibers in an autoclave, introducing pressurized saturated steam into the autoclave to indurate the slurry, and removing the products from the autoclave. Another such method includes placing a molded slurry of the reactive cementitious constituents and reinforcing fibers in an autoclave, introducing pressurized saturated steam into the autoclave to indurate the slurry, simultaneously further indurating and drying the slurry with superheated steam to convert the slurry to a final product, reducing the pressure in the autoclave to atmospheric pressure, and removing of the product.
In practice the principal slurry constituents, i.e. calcareous and siliceous materials, reinforcing fibers and water are mixed to form a slurry which is then molded to impart a predetermined shape to the slurry and final product. The slurry is molded or shaped in any convenient manner. Generally, however, one of two types of molds is employed, i.e. pan molds or filter press molds. In pan molds, the slurry remains in the mold while the cementitious materials are reacted to convert them to a hydrous calcium silicate insulation. A pan mold generally defines a mold cavity of a particular shape and dimension; e.g. a flat rectangular pan is used to form flat ware or blocks, while an arcuate, generally U-shaped mold forms half-section insulation pieces used to form molded pipe covering for insulating pipes, ducts, and the like. The filter press mold generally comprises a perforated molding surface over which the slurry is poured. A perforated mechanical piston, complementary in shape to the mold, compresses the slurry and dewaters it to the point where it is self-supporting. It is with the processing and recycling of the water expressed from the filter press mold at this point in the process that this invention is most concerned.
The filter press molding technique is described in U.S. Pat. No. 2,699,097 and is used to form pipe covering and flat ware.
One method disclosed herein introduces pressurized, saturated steam into a closed system or autoclave in sufficient amount to bring the pressure in the autoclave to 100 to 350 psi at a temperature ranging from 328.degree.-406.degree. F. in 30 minutes or less and preferably within 15 to 30 minutes after the introduction of the steam begins. (The term "pressure" as used herein refers to gauge pressure (in pounds per square inch), i.e. the pressure above that of the atmosphere.)
The slurry is then maintained under this steam pressure in the autoclave for a period of time sufficient to indurate the slurry. After the product has been removed from the autoclave, it can be dried in an auxiliary drier, if required. The drying temperature in the auxiliary drier must be below the decomposition point of the organic fiber reinforcement to achieve best possible strength of the product.
The other method disclosed herein introduces pressurized, saturated steam into a closed system or autoclave in sufficient amount to bring the pressure in the autoclave to 100 to 350 psi in 30 minutes or less and preferably within 15 to 30 minutes after the introduction of the steam begins. (The term pressure as used herein refers to gauge pressure (in pounds per square inch), i.e. the pressure above that of the atmosphere.) After the introduction of the steam, the temperature in the autoclave is raised by heating coils to temperature ranging from about 407.degree. to about 600.degree. F. to produce the superheated steam.
The slurry is then maintained in the autoclave until a predetermined percentage of the moisture (by weight) of the ware has been removed by evaporation into the superheated steam atmosphere. The free moisture is reduced substantially during the cycle, but never drops below 10% by weight of solids during the entire cycle. While continuing to circulate the steam in the system, the pressure in the autoclave is reduced to atmosphere conditions within 60 minutes or less and the final product is removed. The product subsequently can be dried in an auxiliary drier, if required. The drying temperatures in the auxiliary drier must be below the decomposition point of the organic fiber reinforcement, or a brittle product is obtained. The retention of at least 10% by weight-free moisture in the molded slurry allows for simultaneous indurating and drying in an autoclave at temperatures above the decomposition point of the reinforcing organic fibers.
The expressed water resulting from the filter press molding technique results in an accumulation of considerable quantities of waste water. The original gel can contain a weight ratio of water to solids of 12:1 and in the filter press operation this ratio is reduced to 4.5:1. The waste water coming from the filter press step is collected ordinarily in water reclaim pits and is primarily a solution of calcium hydroxide containing suspended matter.
It is highly desirable to minimize the amount of water consumed in the overall manufacturing process, particularly since the consumption of water by industry increasingly is subject to restriction. Further, it is desirable to recycle this filter press waste water through the plant operating system rather than dump it. If, however, the water is to be recycled the calcium hydroxide dissolved therein must be removed. The concentration of calcium hydroxide present is subject to such variations in the process that it is more efficient to remove the calcium hydroxide from the waste water than it is to attempt to compensate stoichiometrically for its presence in water recycled to the original slurrying operation.
Accordingly, a principal object of the present invention is to provide a method for recycling expressed waste water resulting from the filter press operation back into the original slurry step. Other objects, aspects and advantages of this invention will be apparent to those skilled in the art from the following disclosure and appended claims.